The present invention relates to magnetic resonance imaging and, in particular, to radio frequency coils.
Magnetic resonance imaging (MRI) relies on the detection of the magnetic resonance (MR) signal from abundant protons in the volume of interest. A radio frequency (RF) receive coil is a device to effectively xe2x80x9cpick upxe2x80x9d the MR signal from a background of noise for image processing. MR signals induced in a RF receive coil are weak signals due to the very small population difference between the two proton energy states at room temperature. One of the major challenges in RF coil design is to improve the MR signal detection sensitivity.
One of the approaches to improve signal detection sensitivity and/or field of view is to use multiple receive coils as an array. The basic idea is that instead of making a larger and less sensitive coil that covers the entire volume of interest, plural smaller and more sensitive coils are distributed over the volume of interest. Each individual coil picks up signal and noise from a localized volume. With separate detection circuitry, each coil element receives image signal simultaneously. Signals from all coil elements are finally combined and processed to reconstruct MR image for the entire volume of interest.
The principle of MRI involves exciting protons and detecting their free induction decay signals. Each proton possesses a tiny magnetic moment precessing about the static magnetic field. The macroscopic behavior of millions of protons can be represented by a resultant magnetization vector aligning with the static magnetic field B0. A strong RF excitation pulse may effectively tip the magnetization away from B0. The free induction decay of this magnetization is detected in a plane perpendicular to B0. Thus, for maximal signal induction, the normal direction of a receive coil must be perpendicular to the direction of the static magnetic field B0.
Based on the direction of static magnetic field, commercial MRI systems are either horizontal or vertical. So-called co-planar type array coils have proved to be effective for horizontal MRI systems for reasons discussed in the previous paragraph. In a co-planar array coil, surface coils are arranged in a co-planar fashion and distributed over a volume of interest.
In general, such co-planar type surface array coils are not very effective for a vertical system because the condition required for maximal signal induction can hardly be fulfilled. Various modifications to the co-planar designs have been proposed with limited success.
It is known that solenoidal type coils have several advantages for a vertical field system, including its sensitivity, uniformity and its natural fit to various body parts. To successfully implement a solenoidal array coil, one must be able to isolate solenoidal coil elements to prevent them from coupling to each other. This is required because all coil elements in an array coil are to receive signals simultaneously. xe2x80x9cCross-talkxe2x80x9d between different coil elements is un-desirable. Thus effective coil isolation is a major challenge in solenoidal array coil design.
The so-called Sandwiched Solenoidal Array Coil (SSAC) disclosed in U.S. patent application Ser. No. 09/408,506 by Su et al. includes two solenoidal RF receive coil elements, a counter-rotational solenoidal element and a second solenoidal element sandwiched between the two counter-rotational winding sections.
The counter-rotational solenoidal coil element produces a gradient B1 field that has a double-peak xe2x80x9cMxe2x80x9d shape sensitivity profile. The second solenoidal coil element produces a single-peak profile sandwiched between the two peaks of the xe2x80x9cMxe2x80x9d shape profile of the first coil element.
The sensitivity profile of a SSAC is determined by the summation of an xe2x80x9cMxe2x80x9d shape double-peak profile and a centralized single-peak profile generated by the two solenoidal coil elements. To avoid unwanted dark band artifact in the array coil sensitivity profile, the geometric parameters of both coil elements must be set properly.
The uneven-counter-rotational (UCR) coil and its application to a solenoidal array produces a quasi-one-peak sensitivity profile and a null-B1 point, through uneven winding of its counter-rotational solenoidal sections. A second solenoid coil element can be placed near the null-B1 point of the UCR coil to form an inherently decoupled solenoidal array.
A UCR coil based solenoidal array is more versatile than the SSAC based array due to the fact that the former is easier to implement and that an artifact free array signal summation is easier to obtain. However, it still remains difficult to build larger arrays.
A MRI RF coil includes a first solenoidal section, a second solenoidal section, and a third solenoidal section. The first section is between the second and third sections. The first section has a counter-rotational orientation with respect to the second and third sections.